EVALUATION OF HERBIVOROUS DIETARY SPECIALIZATIONS IN AN EARLY EOCENE ECOSYSTEM: A COMBINED APPROACH INCORPORATING PAIRED CARBONATE AND PHOSPHATE STABLE ISOTOPE ANALYSES

Analyses of microwear and tooth shape have suggested that the appearance of new mammalian lineages (such as the perissodactyls) at the Paleocene-Eocene boundary in North America fundamentally changed local ecosystems. Many of the new immigrants likely fed on plant structural parts, a dietary strategy held by only a minor component of North American mammalian faunas prior to the PETM. Stable isotopes in tooth enamel can provide an independent line of dietary evidence, sourcing from the same tooth specimens used for the prior microwear and morphological evaluations. Stable isotope values from both the phosphate (d¹⁸O) and carbonate (d¹⁸O and d¹³C) portions of tooth enamel from two early Eocene mammal assemblages were measured in order to a) compare the dietary strategies of new mammalian immigrants to North America vs. endemic members of the faunas and b) provide an internal check on oxygen isotopic results. Carbonate and phosphate d¹⁸O results show a similar degree of variability, likely stemming from differential reliance on ¹⁸O-enriched leaf water as a proportion of water intake: species identified as likely high-fiber herbivores using microwear and morphological methods tend to have more enriched d¹⁸O values than other members of the fauna. The information available from multiple sources of dietary evidence allows a more complete picture of the sources of stable isotopic variability to be addressed, a powerful tool to the interpretation of isotopic values in both fossil and living systems.